Satellites and other vehicles are in widespread use for various purposes including scientific research and communications. Many scientific and communications missions, however, cannot be accurately fulfilled without the consistent monitoring and controlling of the 3-axis attitude and angular velocity of a vehicle. In many applications, a vehicle must be oriented to transmit signals in a particular direction or to receive signals from a specifically located source. Furthermore, in such applications, the angular velocity of the vehicle must be appropriate to maintain a desired orientation over time. Without accurate control of the 3-axis attitude and angular velocity of a vehicle, the transmission or reception of such signals is hindered and can be unachievable.
Such control requires systems for 3-axis attitude and angular velocity determination, which generally include one or more star trackers or star sensors and a 3-axis gyroscope. During normal operation, the 3-axis gyroscope is used to provide angular velocity information and the star sensors combined with the 3-axis gyroscope are used to provide attitude information. The attitude estimate accuracy is affected by errors in the star sensor and gyro data, and errors resulting from inaccurate knowledge of the star sensor orientation with respect to the body of a vehicle, as defined by the orientation of the 3-axis gyroscope. As these errors are inherent and time varying, it is often necessary to constantly estimate such errors in order to compensate for them.
As a baseline, attitude determination is often performed using data from the star sensors. Orientation of the star sensors can shift over time with respect to the body of a vehicle. These shifts are small, typically approximately several hundreds of arc-seconds in duration and size, and can occur due to thermal effects. Due to the duration and size of the shifts they are difficult to measure on the ground.
The accuracy of the attitude determination varies depending upon the mission and phase of that mission. Certain phases of a mission require that the orientations of the star sensors be known precisely such that errors in the stated orientations can be compensated to provide accurate attitude determination. Thus, occasional measurements of star sensor data are acquired by a ground-team then post-processed to calibrate the star sensor alignments. The star sensor alignments may be moderately or finely calibrated using batch processing of star sensor data.
Ground-based calibration requires a cumbersome process that includes: dedicated orbital passes to gather tracker data telemetered over long periods, approximately one day in length; batch-processing of the tracker data; and vehicle alignment uploads. Ground-based calibration also requires expensive software tools for processing of the tracker data and training of ground personnel to perform the calibration and usage of the tools.
Also, when the vehicle experiences “out-of-contact” periods, such as when telemetry is unavailable, which can often occur with certain satellites the calibration process is hindered. During such periods large blocks of data must be buffered on-board and then transmitted in chunks. This can negatively affect the accuracy of the data and can delay performance of mission tasks.
Thus, there exists a need for an improved system and method of calibrating and estimating the alignments of star sensors.